(a) Technical Field
The present disclosure relates to a thin film transistor array panel using polysilicon as a semiconductor.
(b) Discussion of Related Art
A thin film transistor array panel is used as a circuit substrate to individually drive each pixel in a flat panel display. The flat panel display is, for example, a liquid crystal display or an organic light emitting diode display and has a plurality of pixels.
The liquid crystal display (LCD) includes two panels provided with field-generating electrodes such as pixel electrodes and a common electrode, and a liquid crystal (LC) layer interposed therebetween. The LCD displays images by applying voltages to the field-generating electrodes to generate an electric field in the LC layer, which determines orientations of LC molecules in the LC layer to adjust polarization of incident light.
The organic light emitting diode display (OLED) is a self emissive display device, which displays images by exciting an emissive organic material to emit light. The OLED includes an anode (hole injection electrode), a cathode (electron injection electrode), and an organic light emission layer interposed therebetween. When the holes and the electrons are injected into the light emission layer, they are recombined, thereby emitting light.
Each pixel of the OLED includes two transistors such as a driving transistor and a switching transistor. The current for light emission is driven by the driving TFT and the amount of the current driven by the driving TFT is controlled by the data signals from the switching TFT.
The TFT includes a semiconductor made of amorphous silicon or crystalline silicon. Amorphous silicon is used in displays utilizing glass having a low melting point, since amorphous silicon film can be fabricated at a low temperature.
The amorphous silicon film has low carrier mobility. As a result, the amorphous silicon film may not be well suited for application to a high quality driving circuit of display panels. Whereas, since polycrystalline silicon has prominent electric field effect mobility, high frequency operation, and low leakage current, high quality driving circuits use the polycrystalline silicon.
The electrical characteristics of the TFT using polycrystalline silicon are influenced by the size and the uniformity of grain. In other words, the electric field effect mobility of the TFT is increased with increased size and uniformity of grain. As a result, attention has been directed to the method to form the polycrystalline silicon with increasing the size and the uniformity of grain.
Excimer laser annealing (ELA) and chamber annealing are typical methods for producing polycrystalline silicon. Recently, a sequential lateral solidification (SLS) process deriving lateral growth of silicon crystalline has been proposed.
The SLS technique utilizes a phenomenon that the silicon grains grow laterally with respect to the boundary of a liquid region and a solid region. In the SLS process, the sizes of the grains can be as much as the predetermined widths by controlling the irradiation range and the energy of a laser beam using an optic system and a mask that selectively passes the laser beam.
After the sequential lateral solidification, protrusions are formed on the surface of the polysilicon layer along the grain boundaries due to the grains growing and meeting each other. The protrusions prevent the flow of current, and result in degradation of the characteristics of the TFTs, thereby causing defects such as horizontal stripe or vertical stripe.